One of the best tools for fine-tuning Windows XP is the free TweakUI PowerToy utility from Microsoft. TweakUI digs deep into Windows' settings and can customize its behavior dozens of ways, from how many icons appear on the Alt-Tab dialog to Explorer context menu choices to what your program shortcuts look like. TweakUI's been around forever and we've mentionedithere and there throughout the years at Lifehacker, but it's high time we gave it the full walk-through it deserves. After the jump, take a gander at 15 useful adjustments you can make to your XP system with TweakUI.